1. Field of the Invention
The present invention relates to a dark grey coloured soda-lime glass composed of glass-forming constituents and colouring agents.
The expression "soda-lime glass" is used here in a wide sense and concerns any glass composed of the following constituents (percentages by weight):
______________________________________ SiO.sub.2 60 to 75% Na.sub.2 O 10 to 20% CaO 0 to 16% K.sub.2 O 0 to 10% MgO 0 to 10% Al.sub.2 O.sub.3 0 to 5% BaO 0 to 2% BaO + CaO + MgO 10 to 20% K.sub.2 O + Na.sub.2 O 10 to 20% ______________________________________
This type of glass is used very widely in the field of glazing for buildings or motor vehicles. It is generally manufactured in the form of a ribbon by a drawing or float process. A ribbon of this type may be cut up in the form of sheets which may then be made curved or subjected to a treatment, for example heat treatment, to reinforce the mechanical properties.
2. Description of the Related Art
When speaking of the optical properties of a sheet of glass, it is generally necessary to relate these properties to a standard illuminant. In the present description, two standard illuminants are used; Illuminant C and Illuminant A as defined by the International Commission on Illumination (C.I.E.). Illuminant C represents average daylight having a colour temperature of 6700K. This illuminant is especially useful for evaluating the optical properties of glasses intended for buildings. Illuminant A represents the radiation of a Planck radiator at a temperature of about 2856K. This illuminant represents the light emitted by car headlamps and is essentially intended to evaluate the optical properties of glass intended for motor vehicles. The International Commission on Illumination has also published a document entitled "Colorimetry, Official Recommendations of the C.I.E." (May 1970) which describes a theory according to which the calorimetric coordinates for the light of each wavelength of the visible spectrum are defined in such a way as to be represented on a diagram (known as the C.I.E. trichromatic diagram) having orthogonal axes x and y. This trichromatic diagram shows the location representing the light for each wavelength (expressed in nanometers) of the visible spectrum. This location is called the "spectrum locus" and the light whose coordinates are situated on this spectrum locus is said to possess a 100% excitation purity for the appropriate wavelength. The spectrum locus is closed off by a line called the purple boundary which joins the points of the spectrum locus, the coordinates of which correspond to wavelengths of 380 nm (violet) and 780 nm (red). The area included within the spectrum locus and the purple boundary is that available for the trichromatic coordinates of any visible light. The coordinates of the light emitted by Illuminant C, for example, correspond to x=0.3101 and y=0.3163. This point C is considered as representing white light and on account of this has an excitation purity equal to zero for any wavelength. Lines may be drawn from the point C to the spectrum locus at any desired wavelength and any point situated on these lines may be defined not only by its coordinates x and y, but also as a function of the wavelength corresponding to the line on which it is situated and its distance from the point C with respect to the total length of the wavelength line. From this, light transmitted by a coloured sheet of glass may be described by its dominant wavelength and its excitation purity expressed in percent.
In fact the C.I.E. coordinates of light transmitted by a coloured glass sheet will depend not only on the composition of the glass but also on its thickness. In the present description and claims all the values of the trichromatic coordinates (x,y), of the excitation purity (P), of the dominant wavelength .lambda..sub.D of the transmitted light, and of the light transmittance of the glass (TL) are calculated from the specific internal transmission (SIT.sub..lambda.) of a 5 mm thick glass sheet. The specific internal transmission of a glass sheet is governed solely by the absorption of the glass and may be expressed by the Beer-Lambert law; SIT.sub..lambda. =e.sup.-E.A.lambda. where A.lambda. is the absorption coefficient of the glass (in cm.sup.-1 at the wavelength in question and E is the thickness of the glass (in cm). As a first approximation, SIT.sub..lambda. may also be represented by the formula EQU (I.sub.3.lambda. +R.sub.2.lambda.)/(I.sub.1.lambda. -R.sub.1.lambda.)
where I.sub.1.lambda. is the intensity of the visible light on the first face of the glass sheet, R.sub.1.lambda. is the intensity of the visible light reflected by this face, I.sub.3.lambda. is the intensity of the visible light transmitted from the second face of the glass sheet and R.sub.2.lambda. is the intensity of the visible light reflected to the interior of the sheet by this second face.
In the present description and claims, the following are used.
The total luminous transmission for illuminant A, measured for a thickness of 4 mm (TLA4). This total transmission is the result of integrating the expression: EQU .SIGMA.T.sub..lambda..E.sub..lambda..S.sub..lambda. /.SIGMA..E.sub..lambda..S.sub..lambda.
between the wavelengths 380 and 780 nm, in which T.sub..lambda. is the transmission at wavelength .lambda., E.sub..lambda. is the spectral distribution of illuminant A and S.sub..lambda. is the sensitivity of the normal human eye as a function of the wavelength .lambda..
The total energy transmission, measured for a thickness of 4 mm (TE4). This total transmission is the result of integrating the expression: EQU .SIGMA.T.sub..lambda..E.sub..lambda. /.SIGMA..E.sub..lambda.
between the wavelengths 300 and 2150 nm, in which E.sub..lambda. is the spectral energy distribution of the sun at 30.degree. above the horizon (Moon's distribution).
The selectivity, measured for a thickness of 4 mm (SE4), is defined by the ratio (TLA4/TE4).
The total transmission in the ultra-violet, measured for a thickness of 4 mm (TUVT4). This total transmission is the result of integrating the expression: EQU .SIGMA.T.sub..lambda..U.sub..lambda. /.SIGMA..U.sub..lambda.
between the wavelengths 280 and 380 nm, in which U.sub..lambda. is the spectral distribution of ultra-violet radiation having passed through the atmosphere, as determined as in DIN standard 67507.
The present invention concerns in particular grey glasses with a greenish shade. When the transmission curve of a transparent substance does not vary as a function of the visible wavelength, this substance is described as "neutral grey". In the C.I.E. system, it does not possess a dominant wavelength and its excitation purity is zero. By extension, a body may be considered as grey for which the spectral curve is relatively flat in the visible region but nevertheless exhibits weak absorption bands, enabling a dominant wavelength to be defined and a purity that is low but not zero. Grey glass according to the present invention has an excitation purity not exceeding 10%, preferably not exceeding 6% and a dominant wavelength between 480 and 560 nm, corresponding to a greenish shade.
Grey glasses are generally selected for their protecting properties against the rays of the sun and their use in buildings is known, especially in very sunny countries. Grey glasses are also used in balcony balustrades or staircases as well as for partial glazing in certain motor vehicles or railway compartments. To shield their contents from view, very dark grey glass is mainly employed.
The present invention relates to a selective dark grey glass especially appropriate for use as automobile windows, in particular as rear windows or rear side windows.